


Dark side of the sun

by Askellie



Category: Undertale (Video Game)
Genre: Alternate Universe - Eclipsetale (Undertale), Brainwashing, Captivity, Eclipsetale, Ecto-Vagina (Undertale), Extremely Dubious Consent, M/M, Master/Servant, Non-Consensual Bondage, Psychological Torture, Rape/Non-con Elements, Sexual Servitude
Language: English
Status: Completed
Published: 2020-09-30
Updated: 2020-09-30
Packaged: 2021-03-07 23:47:19
Rating: Explicit
Warnings: Rape/Non-Con
Chapters: 1
Words: 3,704
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/26726149
Author URL: https://archiveofourown.org/users/Askellie/pseuds/Askellie
Summary: Zero was saved and shaped for a single purpose, and only Sun is allowed to determine what he can and cannot be.Based on Skumhuu's Eclipsetale [Dreamswap Variant].
Relationships: Sans/Sans (Undertale), Sun/Moon (Eclipsetalecest), Sun/Zero (Eclipsetale Dream/Cross), Zero/Moon (Eclipsetale Cross/Nightmare)
Comments: 18
Kudos: 134





	Dark side of the sun

**Author's Note:**

  * For [skumhuu](https://archiveofourown.org/users/skumhuu/gifts).



> This fic is a little pre-emptive because unfortunately not much of the Eclipsetale canon has yet been released, but there's been a lot of thirsty theorycrafting over on twitter about the characters and shipping potential and it hits up so many of my kinks, I couldn't help myself.
> 
> Info about Eclipsetale can be found over on Sku's [Twitter](https://twitter.com/skumhuu) and [Tumblr](https://skumhuu.tumblr.com/). It's a variation on the Dreamswap AU, where Sun (Eclipsetale!Dream) is the corrupted one and Moon (Eclipsetale!Nightmare) is just SO DONE with his brother's righteous and overbearing bullshit.
> 
> Zero (Eclipsetale Cross) is formerly one of Sun's subordinates who was given to Moon as a gift, to both protect him and act as a companion to ideally replace Moon's collection of hodgepodge and undesirable (in Sun's opinion) friends. Unfortunately, Zero was a little TOO devoted to Moon and chose to defect to his side after Moon ran away, but his former loyalties to Sun leave him somewhat conflicted. He's also he victim of some pretty heavy mental conditioning, so consider him a very unreliable narrator in this piece.
> 
> Sku has given blanket permission for anyone to play with the Eclipsetale concepts and characters, so none of this is part of the 'canon'. It's just porn. Shameless, shameless porn. 
> 
> [Sun - Eclipsetale Dream](https://twitter.com/skumhuu/status/1302061122430947330)  
> [Moon - Eclipsetale Nightmare](https://twitter.com/skumhuu/status/1302079294353674241)  
> [Zero - Eclipsetale Cross](https://twitter.com/skumhuu/status/1302079294353674241)

Every surface on the inside of the cell casts a searing glow that destroys every shadow. It makes the room look seamless and open even though it’s actually claustrophobically small. Zero can’t even stand up properly, not that he has the will to try. He stays curled up in the corner, face buried in his arms to block out the terrible, maddening white. His screaming despair is trapped beneath a thick knot of choking panic, the terror strangling him to silence.

Trapped and alone, surrounded by unbroken nothingness, it’s a fight to cling to his sanity. Even with the hard wall against his back, it’s a struggle to convince himself that he’s not back in his empty universe. He’s  _ out _ . He’s  _ free _ . He’s  _ saved _ .

(But what if he never was? What if it’s all just been a prolonged fantasy, a trick of his broken mind to escape his empty reality.)

“No, no, no, no-”

His low, desperate mantra gives him something to focus on, a whispered drone of senseless noise that almost covers the spiteful hiss as one of the walls of his tiny prison yawns open. A shadow falls over him, shielding him from the surrounding glare, and Zero’s skull jerks upward. An elegant, long-fingered hand extends towards him, the phalanges dark as ebony and gloriously familiar.

“Oh honey,” Sun coos, peering down at Zero with his gentlest smile. “Come here”

With a broken sound, Zero lunges for the outstretched hand, clinging to it like a lifeline as he’s pulled from the cell and into Sun’s arms. Golden wings fold around him like a comforting blanket as he buries his face into Sun’s chest. Sun is warm and solid and safe, and despite his best efforts to hold it in, Zero can’t hold back a wretched sob of relief.

Sun only holds him tighter, tucking Zero beneath his chin and tenderly stroking his skull. “Shh, you’re okay. I have you.”

Some distant part of Zero’s mind is screeching objections, but that tiny voice is buried beneath the overwhelming desire to let himself be soothed. His breath is coming in raw gasps. His throat hurts, either from the hyperventilating or from all his senseless babbling. The cell was a stifling box of heat, and the circle of Sun’s embrace is even warmer, but Zero can’t bring himself to let go. He makes a keening sound of protest when Sun carefully lifts his face, proffering a small, opened vial and pressing against Zero’s teeth.

“Here, drink this.”

It’s a reflex to obey. He swallows before belatedly recognising the shape of the glass as the kind Linum uses for his potions, but by then the damage is done. The liquid tastes foul, thick with herbs and a dry bitterness that makes him choke, but Sun keeps the vial tilted and forces Zero to drink until it’s finished. It doesn’t clear his head, but he feels marginally calmer. Almost floaty. His vision blurs like his eyelights have diffused, his surroundings distorting into a meaningless smear. Only Sun’s face stays in focus, his beautiful, benevolent smile chasing away the tightness in Zero’s soul.

“There you go,” Sun says, wiping away the streaks of unnoticed tears from beneath Zero’s sockets. His touch is searing, but Zero helplessly leans into them, greedily basking in the contact. Sun gives a soft purr. “Now, I’m sure you’d like to see Moon, wouldn’t you?”

It feels like Zero’s entire being gives a pulse of longing. He stares up at Sun with wide, pleading sockets, nodding frantically. 

Sun looks pleased, which makes some of the tension uncoil from Zero’s bones. He was sure the next time he saw Sun, the guardian would be disappointed or angry. If he’s upset by Zero’s defection, it doesn’t show. He turns his back as easily as he always has, expecting Zero to fall into step behind him like his loyalty has never been in question.

Zero doesn’t disappoint him. The brief, wild thought of escape slips away before it can even take form. Sun is already leading him where he wants to go, and taking him away from the horrible white box. That’s all that matters.

(Is that all that matters? He thinks there’s more, but it’s buried beneath the calm along with his panic. The weight of his locket and Moon’s symbol feels heavy against his sternum.)

Normally when Zero accompanies Sun, it’s to keep watch and guard him against potential threats, but Zero can’t seem to stay focused on anything except Sun himself. The bright, rich colors of his wings and clothes seem mesmerising after the blandness of the cell. Zero’s fuzzy vision makes him look positively ethereal, a luminous creature of light and heat drawing Zero in like a moth to a flame.

Sun pauses before a thick, sealed door that’s practically humming with the restrictive wards scrawled into it. He shoots a small, fanged smile over his shoulder at Zero. “Be sure to behave yourself, Honey.”

Zero gives a confused, jerky nod, holding himself stiffly as the door opens up to reveal another bright, shadowless chamber. Zero barely manages a glance at the stark walls before he’s forced to avert his eyes. An uneasy sweat trickles down his spine, and his body feels leaden like his soul is being held down with blue, but with difficulty he manages to follow Sun whose long legs carry him with ease and swiftness into the room. 

“Hello, Moonbeam!” Sun greets with inane cheer. “I brought you a gift.”

Zero isn’t prepared to be swept forward by one of Sun’s wings, almost staggering from its negligent strength. Only Sun’s swift reflex to catch him humerus keeps Zero on his feet, steadying him with an unrelenting grip that refuses to let him turn away from the terrifying whiteness of the room. Unlike Zero’s cell, this chamber is much bigger, and it’s not completely empty like he thought. There’s a long golden chain hanging from the ceiling, and dangling from the end by his shackled wrists is Moon, who looks equally stunned to see him. 

Zero lurches towards him like metal drawn by a magnet, but he’s barely taken a step before Sun orders, “Stop.” 

Against his will, his body obeys, halting in place even though he’s trembling with the urge to reach out. Moon looks small and helpless, stripped of the dark colors he favours and left only in a pale shift of cloth that’s practically see-through. Sun clearly isn’t taking any chances that Moon might draw power from shadows falling even beneath his own clothing. Zero can see the outline of every bone beneath the translucent cloth, the inadequate covering somehow more salacious than bare nudity would be. He looks unharmed but furious, his sockets blazing with anger as he jerks against the shackles. There’s a muzzle sealed over his lower jaw and mouth, muffling his voice, but Zero can hear the urgent note of his plea.

Before he can even think to move, Sun’s hand comes down on his shoulder, his grip firm but not painful. Even so, it burns through his shirt. A hot, dangerous flush runs through him -- Sun’s magic caressing his body in silent warning -- leaving his legs shaking as Sun smoothly transitions his grip to curl around Zero’s shoulders and guide him forward.

“No need to look so angry, Moonbeam. I brought you one of your friends, just like I promised.”

Moon’s reply is indecipherable, but Zero can still tell it’s some sort of obscenity. Sun’s expression twitches before smoothing back into a gentle smile. “You’re angry with me, but that’s fine.This is what’s best for you, even if you don’t appreciate it right now.”

The gagged snarl Moon makes is vicious but impotent. Helpless. There’s a sour taste of discontent in the back of Zero’s throat. His shoulders feel painfully taut, his hands curled into tight fists. Sun looks between them and heaves a dramatic sigh, shifting his weight to lean heavily against Zero’s back.

“I know it’s hard to see him like this,” he tells Zero, drawing his attention away from Moon’s outraged expression with a tender hand on his cheekbone. “But it’s dangerous out there for him. This is the only way to keep my brother safe. That’s the most important thing, isn’t it, Zero?

Zero opens his mouth, but it’s like all the words have flown from his mind in the face of Sun’s blood-red stare. He still feels disoriented by the sensory whiplash, his stimulus-starved senses now oversaturated with the intensity of Sun’s presence. Overwhelmed, most of Sun’s words don’t even register. Only the question sticks, because it demands a response that Zero needs to give to keep Sun happy.

The most important thing...is keeping Moon safe? That seems like a safe agreement, so he nods. It must be the right answer, because Sun beams at him. 

“That’s right. You’ll help me keep him safe and happy, won’t you?

“Yes,” Zero rasps, his body thrumming like the plucked string of a perfectly played instrument. Sun draws him close enough to share a breath, and Zero lets his sockets flutter shut in familiar surrender as Sun lays claim to his mouth with a warm purr of approval.

Moon makes a splutter of sound, distant and unimportant. Nothing else can matter when Sun is kissing him. The guardian demands every inch of his attention, of his devotion, of his being. Zero can’t afford to be distracted, not with Sun’s hands cupping his jaw, long fingers carefully encircling his cervical vertebrae like a noose. Sun’s tongue is a searing invader, exploring Zero’s mouth with the confidence of one reclaiming familiar territory. Zero can’t hold back the soft, breathless sounds he’s making, and he flushes hot at the knowledge that Moon is listening.

When Sun finally pulls back, Zero is dizzy and swaying on his feet. Sun looks like a satisfied predator in the aftermath of a meal. 

“That’s right. You know where you belong, don’t you?”

Zero nods thoughtlessly, clinging to Sun’s shoulders. He can feel the pulse of his soul echoing against Sun’s ribs, but there’s no answering beat from Sun’s own chest. That’s always a slightly unnerving realisation; that his unusual soul doesn’t pound like a mortal monster. Sun’s smile stretches even wider.

“And you know what your place is.”

Zero lowers his gaze, his face. As pleasant as it is to be in Sun’s arms, that’s not where he belongs. That place is reserved for Moon, as it should be. “Yes.”

He takes a small step back and falls smoothly to his knees. It’s a familiar motion, grown comfortable with repetition, but for some reason Moon makes a sharp, disruptive hiss; an objection. Zero glances up to see him twisting against the shackles, his expression trying to convey some complicated message that Zero can’t decipher.

Sun clicks his tongue disapprovingly. “Aww, Moonbeam, are you feeling jealous? Don’t worry, I didn’t forget you.”

Leaving Zero on his knees, Sun reaches out to Moon who cringes but can’t avoid his touch. Sun wraps himself around his smaller twin, his dark phalanges stark against the pale luminosity of Moon’s bones and the flimsy shift of cloth. He nuzzles into the crook of Moon’s neck, breathing deeply as he confesses with rare sincerity, “I missed you so much.”

There’s a flicker of light from Sun’s hand, and Zero jolts in alarm. He knows Sun’s magic doesn’t work on Moon the way it does on ordinary monsters. There’s no chance of him turning into salt, but there’s a crackle of sound like the snap of electricity. Moon convulses like he’s been shocked, but the sound he makes shoots a bolt of heat straight through Zero’s soul. It’s a whimper that borders on a moan, less a cry of pain than a startled gurgle of need.

When Sun moves his hand, there’s a mark on Moon’s ribs; a small, dark bruise like the imprint of a kiss. The bone around it flushes with the vibrant purple hue of Moon’s magic. Zero can’t help staring at it, thinking helplessly of how sensitive it must be. Sun strokes the inflamed rib carefully, tracing lovingly over the mark.

“I need you, Moonbeam. Nothing matters without you.”

Moon’s expression contorts with conflict, but his distress goes soft and unfocused as Sun deftly slips a hand beneath his flimsy covering to touch his ribs directly. Zero knows those hands feel; deviously honing in on every plane of sensitivity, squeezing and scratching and twisting bones against the magic that holds them in place to create an exquisite ache at the joints. He feels a warmth blooming in his own body, a pavlovian response to watching the spider-like dance of Sun’s fingers moving over bones even if they’re not his own. His breath hitches more loudly than he intends, and both twins turn to stare at him. Sun’s expression is full of knowing smugness while Moon is wearing a bright blush to match the one Zero can feel on his own cheeks.

“Zero,” Sun purrs. “Do you want to help my brother?”

Zero isn’t sure what ‘help’ means in this context. Probably not what he thinks he does, but if he takes the words at face value the only possible answer is an earnest and pleading, “Yes.”

The crook of Sun’s finger feels like a hook tugging at Zero’s soul. “Then come here.”

Mindful of his place, Zero crawls across the floor until he’s kneeling at Moon’s feet. He can’t bring himself to look up into Moon’s expression -- it’s too raw, too much to see the flush of arousal on Moon’s face. Keeping his gaze level is equally fraught. Moon’s pubic symphysis is dark and discolored, not from Sun’s touch but from the thick buildup of his own stimulated magic.

“I trained you to be good with your mouth,” Sun reminds him, as though Zero could forget all the hours spent learning how best to utilise his tongue and teeth for another’s pleasure. “Tell me, did my brother ever make use of your services?”

Zero cringes slightly, feeling embarrassed even though Sun has always insisted that he shouldn’t be. He should be proud to offer his body to the guardians, proud to serve them in any way he can. “No.”

“Did you never offer?” Sun tuts reprovingly. “You’re been negligent in your duties. You should make up for that.”

Zero quails from the implicit disappointment in Sun’s voice. It was never his intent to deny Moon anything. He would have willingly given his beloved charge whatever he asked for -- his body and his affection included. It was simply that Moon had never made such a need known. If anything, his reaction to Silver’s overtures just demonstrated that Moon was uncomfortable with physical intimacy. He seemed to prefer simply basking in Zero’s silent, watchful presence, any instances of touch kept careful and brief.

But he can see Moon’s body is twitching and flushed with arousal. Zero can smell his desire and wants nothing more than to ease it. He’s ready for this. He’s made for this, to be Moon’s perfect companion, giving him everything he needs. This is his purpose. 

Still, he sits back on his heels, waiting for the order, just as Sun trained him “Master, can I-?”

“Yes,” Sun murmurs, pulling the hem of the shift up past Moon’s hips, exposing his pelvis. “Pleasure him. Show him what a gift you really are.”

The praise makes Zero weak with gratitude. He leans forward, mouth open to eagerly taste the crest of Moon’s pubic symphysis. Moon’s magic isn’t formed but it’s thick enough to buzz on Zero’s tongue, sweet and syrupy with a tingling aftertaste like a blackberry wine. With reverent devotion, Zero laves his tongue across the bones, paying special attention to the places that make Moon quiver and whimper.

He can hear Sun murmuring soft encouragements to Moon. Zero can’t make out the words, but the comforting heat of his voice lulls Zero in a near trance, the familiarity of the act taking over as his mind goes soothingly blank. He keeps his hands at his sides (Sun never liked any attempt to cling or hold him down) but uses both his tongue and teeth to their fullest. He lets the points of his fangs scrape along the bone to heighten sensitivity before retracing the same areas with the plush surface of his tongue. Moon’s magic buzzes against his senses, struggling to resist his efforts

“So stubborn,” Sun murmurs. “Give me what I want, brother.”

He reaches down to where Zero’s tongue has made the bone slippery with saliva, and his dark phalanges glow with a burst of heat that makes Moon screech, his whole body jerking. With an audible snap his magic coalesces into place, the ectoflesh of his newly formed pussy a swirling galaxy of dark purple filling up the space inside his pelvis.

“Good boy, Moonbeam. You’re doing so well,” Sun breathes. His fingers dip between his brother’s folds, making Moon whimper. Zero watches, transfixed as Sun’s dark phalanges gently part the outer lips, revealing a bright stripe of green along Moon’s inner slit, a vibrant slash of color hidden like a secret. Lasciviously, Sun keeps Moon spread open and exposed, bright and bare in Zero’s face.

“Lick,” Sun orders, and Zero lurches to obey, his mouth already watering. He buries his nose against Moon’s pussy, nuzzling devotedly as he laps at the first trickle of slickness leaking out of him. 

Moon is writhing, moaning senselessly with noise as he’s mercilessly ravaged by Zero’s mouth and Sun’s fingers. They work in tandem with practiced teamwork, Sun pinching wickedly at Moon’s clit before allowing Zero to tenderly lick and sooth the tender nub. When Sun’s phalanges curl into Moon’s entrance, Zero’s tongue pushes in alongside them, fighting for space in his tight passage. Very tentatively, Moon’s hips start to rock against Zero's face, finding a shaky rhythm between the aggressive thrust of Sun’s fingers and the gentler curl of Zero’s tongue.

Zero’s chin is dripping with Moon’s slick and his own saliva. His skull is full of Moon’s smell and taste, a sweet musk that’s intoxicating and wonderful. He rarely allows himself to want things, and this is far more than he deserves, but he feels deliriously happy to have been granted it even once. Moon is finally making use of him the way he was always meant to. It’s more than Zero dared to hope for.

Moon is quaking, his gagged voice coming in high, strangled pants. It’s different from Sun, who’s very quiet and controlled, but Zero knows immediately that it’s a signal of the oncoming climax. Fervently, he eases back and flicks his tongue against Moon’s clit in rough, firm strokes that make the guardian’s lithe body spasm fitfully before he goes taut and still. The messy slide and thrust of Sun’s fingers slows to a halt as they simply hold Moon, bracing him through the insensible wails as he reaches the wild peak of his orgasm. Zero is breathless from the effort, his jaw aching, but he leans against Moon’s femurs feeling fulfilled; utterly content. 

Until he feels something drip onto him from above, a wet splatter falling on his skull. For a moment, Zero mistakes it for the slick arousal now oozing down Moon’s femurs, but when he looks up it’s not into a blissful expression of satisfaction but into a pinched face of tears.

Moon is crying. The brief surge of pride Zero felt at bringing Moon to completion crumbles immediately into dust.  _ Moon is crying _ . His tears are falling down on Zero’s skull. His eyes are open but dull, full of misery and defeat. 

“M-Moon…?” He reaches up to touch Moon’s face, only to have his hand intercepted by Sun. 

“You’ve done enough, Honey.” He says it kindly, but the words feel like a slap. Zero glances bewilderedly between the twins, unable to understand why Moon look so sad when Zero did everything right, everything he was trained to,  _ everything Sun told him to _ -

Zero’s skull is full of static, his thoughts swirling in a murk of confusion. He feels helpless in the face of Moon’s misery, but thankfully Sun seems to know what to do. His voice is sweet and coaxing as he carefully smooths away the streaks of Moon’s tears.

“Moonbeam, don’t cry. You were perfect.” 

He doesn’t look concerned by Moon’s unexpected shift of mood. If anything, he seems pleased, leaning close to murmur something in Moon’s ear that Zero isn’t privy to. Whatever it is makes Moon’s sockets go wide, eyelights flaring brightly with some emotion Zero can’t interpret. 

Smiling, Sun plants a kiss on his brother’s temple before turning away. A curt gesture brings Zero to his feet, stumbling clumsily in his wake like he’s a puppet on strings. Everything in him is screaming to stay with Moon, but guilt and confusion deprive him of agency. All he can do is follow Sun out of the cell, feeling his soul twist when the door seals shut behind them.

Zero’s head hurts. His chest hurts, like his soul is being squeezed too tight. He’s not at all prepared when Sun immediately rounds on him in the corridor, cornering him against the wall.

“Now, Honey, we need to talk.”

It takes only a light push on Zero’s shoulder for his shaky legs to collapse beneath him, sending him back to his knees. He blinks dazedly up at Sun, whose inscrutable smile gives him nothing to work with.

“It seems Moon wasn’t wholly satisfied with your efforts. I think you need more training.” A hand on Zero’s skull forces him to lower his gaze towards the tented fabric at Sun’s groin. “I’ll give you this chance to redeem yourself. Show me you’re still worthy to serve my brother.”

Zero anxiously wets his teeth, still tasting Moon’s sweetness on his tongue. His hands are trembling, and he’s terrified of being proven a disappointment, but at least like this knows what’s expected of him. He doesn’t have to try and think past the tangle of thoughts in his head. All he has to do is follow orders.

Wordlessly, he reaches out for the golden buckle of Sun’s belt. 


End file.
